In telephone systems, line circuits serve as an interface between communication lines connected to subscriber sets and the telephone switching network. Traditionally, the line circuit has included a battery feed to energize the subscriber set. It has been recognized that the effect of longitudinal currents, which are frequently induced in a subscriber loop because of nearby electrical power lines, can be minimized when the battery feed is capable of floating with respect to ground. Known line circuits have implemented a floating battery feed by using a transformer to isolate the subscriber set from the office DC source.
In the telephone subscriber line circuit disclosed in U.S. Pat. No. 4,056,691 issued to L. Freimanis et. al. on Nov. 1, 1977, a switching-mode, flyback power converter which supplies current to a telephone loop, operates in a constant-power mode for low loop impedances but otherwise operates in a constant-current mode. Although representing a significant advance over previous circuits, the Freimanis circuit has a number of disadvantages. The control circuitry which alternately connects and disconnects the office DC source to the primary winding of a converter transformer at a rate dependent on the loop impedance to achieve constant-current operation is relatively complex. Further since that control circuitry is energized by the office DC source, e.g., a 48-volt source, its implementation using presently available, low-voltage integrated circuit technology is not possible. In view of the foregoing, a recognized problem in the art is providing a floating battery feed circuit of reduced cost and complexity having a control circuit which operates at a voltage substantially lower than 48 volts.